1. Field of the Invention
The present invention relates to a thermal printer having a thermal head.
2. Background Information
In thermal printers, the printing density is generally known to become denser as the temperature of the heating elements of the thermal head increases. Furthermore, the temperature of the heating elements is controlled by controlling the time during which electric power is transmitted to these heating elements. Moreover, each of the heating elements corresponds to a dot of the printed image, and the time during which the electric power is transmitted to these heating elements is controlled for each heating element. These times are set forth beforehand in the form of tables 121P as shown in FIG. 6. In the time table 121P used in a known thermal printer, electric power is not provided to the heating elements that correspond to dots having zero (0) printing density, i.e., dots that are not to perform the printing (printing blank).
Here, FIG. 7 shows a diagram used to illustrate the problems encountered in known thermal heads. FIGS. 7(a) and (b) show respective examples of black and white printed images. Furthermore, in FIG. 7, the direction of printing is indicated by an arrow for the purposes of illustration.
In this thermal printer, transmission of power is stopped to the heating elements of which the printing density is 0. Accordingly, in the case of images in which there is a continuous blank portion (i.e., a continuous non-printed state) as shown, for example, in FIG. 7, the time period during which no power is provided to the heating elements continues for a considerable amount of time. During such period, the head temperature decreases. As a result, when black portions are to be printed subsequently, there are cases where the head temperature is insufficient so that the desired density cannot be obtained.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved thermal printer that overcomes the problems described above. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.